System for reproducing four channel signals recorded on a disc record

ABSTRACT

As described herein, sound information comprising four discrete audio signals is recorded on a single groove, two-channel record disc. To this end, two of the four signals are added to produce a sum signal and subtracted to produce a difference signal. The difference signal is employed to amplitude modulate a carrier signal and the upper sideband thereof is added to the sum signal and applied to one input terminal of a recording transducer. The transducer modulates one channel of the record groove in response to the sum signal and the upper sideband signal applied thereto. A similar operation is performed upon the remaining two signals, and the transducer modulates the second channel of the record groove in response to the sum signal and the upper sideband applied thereto. In reproduction, the two composite signals picked up from the two channels of the record groove are each separated into their respective components, the separated modulated carrier signals are demodulated to recover the two difference signals, and the demodulated difference signals are separately matrixed with their corresponding sum signals to recover the four original audio signals for reproduction on respective loudspeakers placed at predetermined positions in a listening area.

' United States Patent Bauer et a1.

[451 May 13, 1975 Assignee: CBS Inc., New York, N.Y.

Filed: Apr. 30, 1974 Appl. No.: 465,467

Related US. Application Data Division of Ser. No. 428,294, Dec. 26,1973, which is a continuation of Ser. No. 169,219, Aug. 5, 1971,abandoned.

References Cited UNITED STATES PATENTS 2/1959 Dauguet 179/15 BT 12/1962Minter 179/100.4 ST 3,401,237 9/1968 Takayanagi.. l79/100.4 ST 3,686,4713/1972 Takahashi l79/l00.4 ST

FOREIGN PATENTS OR APPLICATIONS 1,196,711 7/1965 Germany 179/l00.1 TD

OTHER PUBLICATIONS Burstein, News For The Audiophile, Radio-Electronics,pages 3839, Apr. 58.

Primary Examiner-Raymond F. Cardillo, Jr. Attorney, Agent, orFirm-Spencer E. Olson [57] ABSTRACT As described herein, soundinformation comprising four discrete audio signals is recorded on asingle groove, twochannel record disc. To this end, two of the foursignals are added to produce a sum signal and subtracted to produce adifference signal. The difference signal is employed to amplitudemodulate a carrier signal and the upper sideband thereof is added to thesum signal and applied to one input terminal of a recording transducer.The transducer modulates one channel of the record groove in response tothe sum signal and the upper sideband signal applied thereto. A similaroperation is performed upon the remaining two signals, and thetransducer modulates the second channel of the record groove in responseto the sum signal and the upper sideband applied thereto. Inreproduction, the two composite signals picked up from the two channelsof the record groove are each separated into their respectivecomponents, the separated modulated carrier signals are demodulated torecover the two difference signals, and the demodulated differencesignals are separately matrixed with their corresponding sum signals torecover the four original audio signals for reproduction on respectiveloudspeakers placed at predetermined positions in a listening area.

2 Claims, 3 Drawing Figures Low PHASE DE-EMFHASIS C EpuAuzms cincuir 6266 lLg+Lgl+ moo. CARRIER I lClRCUlT 1 i J 1 55 5; J PREAMPLlFlER MATRIX-46 5 8 6,4 l 7} men AM DE-EMPHASIS fg nerecron h ciacuir 68 LOW PHASEDE-EMPHASIS 44 45 4B C eounuzms CIRCUIT 62' 1 CIRCUIT I 1 I 53' g, J RfPREAMPLIFIER MATRIX R 5 4 :7) b (a. R t MOD. CARRIER mg: m DEEMPHASISHLTER DETECTOR cmcunr 1 SYSTEM FOR REPRODUCING FOUR CHANNEL SIGNALSRECORDED ON A DISC RECORD This is a division of application Ser. No.428,294, filed Dec. 26, 1973, which, in turn, is a continuation ofapplication Ser. No. 169,2l9, filed Aug. 5, l97l, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to stereophonic discrecording systems and, more particularly, to a new and improvedrecording system adapted to record four independent channels of audioinformation on a two-channel disc record.

In systems for recording stereophonic information comprising two signalsin a single groove record disc. it is customary to modulate the twochannels (left and right) of the record groove in two perpendiculardirections. Thus, the groove variations in one channel represent onefunction of the two signals and those in the other channel representanother function of the two signals. Typically, the groove is cut withmodulation in each channel of the groove representing each signal andwith lateral modulation representing the sum of the signals and verticalmodulation representing the difference between the signals.

Since the commercialization of stereophonic phonograph soundreproduction in about 1958, efforts have been made to record three ormore signals on a twochannel record for the obvious reason that thesound distribution of the reproduced information will improve as thenumber of signal sources increases. For example, three channel discrecording systems have been proposed in which a selected fraction of athird signal corresponding to the sound detected by a centrally disposedmicrophone is added to the two signals corre sponding to the sounddetected by spaced left and right microphones to record a dependent sum,or center, channel on the stereophonic record. Alternatively, it hasbeen proposed to subtract a selected fraction of the third signal fromeach of the other two signals and to record a dependent differencechannel on the stereophonic record. However, such a difference channelis not compatible with monophonic players because the vertical groovemodulation cancels for monophonic reproduction, nor with stereophonicplayers which produce an unnatural sound during reproduction.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention to pro vide a method and apparatus for recording four discretechannels of audio information on a two-channel disc record, and forreproducing the four discrete channels upon playback of the record.

Another object of the invention is to provide a system for recordingfour discrete channels of audio information on a two-channel disc recordwhich may be reproduced on a conventional stereophonic phonographwithout loss ofany sound contained in the original four signals.

These and other objects of the invention are accomplished by processingfour discrete audio signals and modulating each channel of a two-channeldisc record of the conventional 45-45 modulation type in accordance withseparate functions of two of the signals. For convenience ofdescription. the four discrete signals correspond to the positions in alistening area at which tour loudspeakers would be placed torespectively reproduce the four signals; that is, they may becharacterized as left front (L right front (R left back (L and rightback (R,,). In particular, selected pairs of the audio signals, forexample, L; and L and R, and R,,, are added to produce sum signals andsubtracted to produce difference signals. The difference signals areemployed to amplitude modulate carrier signals and the resultant carriersignals are combined with their respective sum signals. The combined sumsignals and amplitude modulated carrier signals are then recorded on theseparate channels of a two-channel stereophonic record.

In a preferred embodiment of the invention, only the upper sidebands ofthe difference signal modulated carrier signals are combined with theirrespective sum signals and recorded on the separate channels of therecord. To compensate for the envelope distortion of the single sidebandcarrier signals, envelope processing circuits are provided to process orpredistort the difference signals prior to developing the singlesideband. In addition, equalizing networks are provided to maximize themodulation in the upper frequency ranges of the difference signals.

In a preferred embodiment of a record reproducing system, the sumsignals are separated from the difference signal modulated carriersignals, the difference signals are detected, and the sum and differencesignals are again added and subtracted to produce four audio signalscorresponding to the original four channels for application to fourappropriately positioned loudspeakers.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawing:

FIG. 1 is a block diagram illustrating a preferred embodiment of arecording system arranged according to the invention for recording fourdiscrete channels of audio information on a two-channel disc record;

FIG. 2 is a diagram illustrating the frequency distribution of the sumand the single sideband carrier signals recorded on one channel of atwo-channel stereophonic record; and

FIG. 3 is a block diagram illustrating a preferred embodiment of areproducing system arranged according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In a preferred embodiment of afour-channel recording system arranged according to the presentinvention, as shown in FIG. I, a pair of matrix circuits l0 and 10'receive four audio signals from a suitable four-channel source, such asa master tape, (not shown) via input terminals l2, l4 and l2, 14',respectively. As illustrated, the four input signals are designated leftfront (L left back (L,,), right front (R and right back (R and eachcorresponds to the location in a listening area in which they are to bereproduced. The processing circuitry for the L; and L,, signals will bedescribed in detail. As will be apparent hereinafter, the L, and Lsignals are recorded in the left channel of a sterophonic record and theR, and R signals are recorded in the right channel. The components ofthe processing circuitry for the R; and R signals corresponding to thecomponents of the L; and L,, signal processing circuitry are designatedby prime numbers.

Within the matrix circuit 10, which may be of cnventional constructionand, accordingly, need not be described herein, the L, and L signals areadded together to supply a sum signal (L,+ Li.) to a channel 16 andsubtracted to supply a difference signal (L, w L,,) to a channel [8. Asdescribed hereinafter. the sum signal (L,+ L is recorded in the leftchannel of a stereophonic record and the upper sideband of a carriersignal modulated by the difference signal (L, L is also recorded in theleft channel of the stereophonic record. In like manner, the sum signal(R,+ R1,) developed in the matrix It) and the upper sideband of acarrier signal modulated by the difference signal (R, R1,) are recordedin the right channel of the stereophonic recordv The sum signal '(L,L,,) is applied via a phaseequalizing circuit 19 to a buffer amplifier20, whereas the difference signal (L, L is applied initially to anequalizer circuit 22. The phase equalizer circuit 19 is provided tomaintain coincidence between the sum signal and the difference signal,which is phase-altered by reason of the processing to which it issubjected, as will be apparent hereinafter.

The equalizer circuit 22 attenuates the components of the differencesignal (L, L,,) ranging in frequency from about 20 Hz to about 1 kHz tocompensate both for the fact that some of the lower sideband signal istransmitted to the output of the processing circuitry because of themethod employed for producing single sideband. The signal-to-noise ratioof the composite signal is improved by pre-emphasizing the highfrequency components of the difference signal (in which the spectralenergy is less than for mid-range frequencies) and correspondinglydeemphasizing the signal during playback so that a flat frequencyresponse is attained and the record noise subsequently attenuated. Themethod utilized to generate single sideband is to pass a double sidebandsignal with carrier through a high-pass filter having a cutoff frequencyequal to the carrier frequency. Consequently. the audio level for lowfrequencies must be attenuated with respect to the high frequencies sothat the modulation percentage in the double sideband region is the sameas in the single sideband region for constant level input. These signalsranging in frequency from about 17 kHz to 20 kHz will be transmitted,albeit with attenuation.

From the equalizer circuit 22 the difference signal is supplied to anenvelope processor circuit 24 which includes an amplitude modulator 26,a highpass filter 28, a full wave detector and an inverter 32. Thepresent invention utilizes a single sideband modulator to record thedifference signal (L, L,,) on the same channel (left) of thestereophonic record groove that the sum signal (L,+ L,,) is recorded. Asis understood in the art. when a sideband is removed from an amplitudemodulated carrier signal to develop a single sideband signal withcarrier. the envelope of the single sideband signal is in the shape of apoid" and thus represents a distorted replica ofthe difference audiosignal. In order to compensate for such distortion of the envelope ofthe single sideband signal. it is a feature of the present invention topredistort the difference signal (L, L,,) in the processor 24.

Within the processor 24, therefore. the difference signal (L, L,,) isapplied to one input terminal of the amplitude modulator 26 wherein thesignal modulates a carrier signal having a frequency of 20 kHz. Thedeveloped modulated carrier signal is then applied to the highpassfilter 28 having a cutoff frequency of 20 kHz which rejects at rapidrate frequencies below 20 kHz and passes without attenuation the uppersideband of the modulated carrier signal. The transmitted sidebandsignal is then applied to the full wave detector circuit 30 whichcomprises, for example, a full wave rectifier and lowpass filter and,accordingly. detects only the envelope of the transmitted sidebandsignal. As above described. the detected signal constitutes a distortedreplica of the difference signal (LI- L1,).

The inverter 32 inverts the detected and distorted difference signal andapplies the inverted signal to one input terminal of an amplitudemodulator 34. composing the first component in a single sidebandmodulator 36. In the modulator 34, the inverted and distorted differencesignal modulates a 20 kHz carrier signal applied to the other inputterminal of the modulator, A highpass filter 38 having a cutofffrequency of 20 kHz transmits only the upper sideband of the differencesignal amplitude modulated carrier signal. By virtue of the inherentdistortion in the modulation and following process, the envelope of thetransmitted upper sideband signal is distorted with the result that theprevious distortion is cancelled and the envelope of the resultantsignal is an exact replica of the difference signal (L,

From the filter 38, the upper sideband signal with carrier is suppliedto a buffer amplifier 40 having its output terminal connected to oneinput terminal of a summing network to the other input terminal of whichthe output terminal of buffer amplifier 20 is applied. In the summingnetwork 41, the sum signal (L, L,,) and the (L, L,,) single sidebandsignal with carrier are added together and supplied to one inputterminal of a recording transducer 42. In this case. the transducer 42is arranged to modulate the left channel of the record groove by meansof a stylus 43 in response to the sum signal (L,+ L,,) and the (L, L,,)single sideband signal with carrier supplied thereto.

ln like manner, the sum signal (R, R1,) and the upper sideband signalwith carrier having an envelope representative of the difference signal(R, R,,) are added together in a summing network 41' and then applied tothe other input terminal of the recording transducer 42. The transducer42 is arranged to modulate the right channel of the record groove inaccordance with the sum signal (R, R and the (R, R single sidebandsignal with carrier supplied thereto.

Referring now to FIG. 2, there is shown the frequency distribution ofthe sum signals (L,+ L,,) and the difference signals (L, L,,). The sumsignal is recorded as a regular audio signal in the respective channelof the record groove and the difference signal is recorded as singlesideband modulation with carrier in the same channel of the recordgroove. The total bandwidth ex tends from 30 Hz to 35 kHz. Thedistribution of the frequency components of the right channel signal isiden tical to that shown for the left channel.

ln a preferred embodiment of a four-channel record reproducing systemarranged according to the present invention. as shown in FIG, 3, atwo-channel disc phonograph pick-up transducer 44 includes a stylus 45that responds to the modulations in the left and right channels of arecord disc recorded in the above-described manner to supply the sumsignal and the (L,* L,,) single sideband signal with carrier. and thesum signal (R, R,,), together with the (R,- R single sideband sig nalwith carrier, along a pair of conductors 46 and 48, respectively, to apair of preamplifiers 50 and 52, respectively. Like the recordingsystem, the components of the processing circuitry for the left andright channels are identical, and accordingly, only the processingcircuitry for the left channel information will be described herein. Thecorresponding components in the right channel are designated by primenumbers.

The preamplifier 50 amplifies the sum signal (L; L and (L, L singlesideband signal with carrier and applies the amplified signal to both alowpass filter 53 and to a highpass filter 54. The filters 53 and 54separate the sum and single sideband signals and supply the separatedsignals to a phase equalizing circuit 56 and to an AM detector circuit58, respectively. The phase equalizing circuit 56 is provided tomaintain coincidence between the sum signal and the (L,- L,,) singlesideband signal with carrier which is altered in phase by virtue of theadditional circuits to which it is subjected, as will be apparenthereinbelow. From the circuit 56, the sum signal is applied by way of adeemphasis circuit 60, conventional in stereophonic record reproducingsystems, to one input terminal of a matrix 62.

In the AM detector circuit 58, which may comprise, for example, a diodenetwork and a lowpass filter, the envelope of the (L; L single sidebandsignal with carrier is detected, the detected signal being an exactreplica of the (L, L signal produced by the matrix in the recordingprocess (FIG. 1). The difference signal (L, L is then applied by way ofa de-emphasis circuit 64 to the other input terminal of the matrix 62.In the matrix 62, the sum signal (1 L is combined with the detecteddifference signal (L, L in the conventional manner to reproduce at twopairs of output terminals 66 and 68 the left front (L signal and theleft back signal (L,,). The right front (R,) signal and the right back(R signal are reproduced in like manner at the two pairs of outputterminals 66 and 68' of the matrix 62. Although not shown, these signalsmay then be supplied through power amplifiers to four spacedloudspeakers positioned at the left front, right front, left back andright back corners of the listening area, so that the spatial relationof the original four-channel sound is reconstructed during reproduction.

In operation, the matrices l0 and 10' receive signals Ly, L R, and R andconvert the signals into sum signals (L;+ L,,) and (R;+ R and differencesignals (L, L and (R, R respectively. The sum signal (L, L,,), togetherwith a (L, L,,) single sideband signal with carrier, is recorded in theleft channel ofa stereophonic record and the sum signal (R,+ R togetherwith a (R;-- R,,) single sideband carrier, is recorded in the rightchannel of the record. In the reproduction of the record, the sum anddifference signals in each channel are separated and the differencesignal detected. Thereafter, the signals are combined in the matrices 62and 62' and reproduced as the original four audio signals, L L R, andR,,.

Although the invention has been described herein with reference to aspecific embodiment, many modifications and variations therein will bereadily apparent to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention as defined by the following claims.

We claim:

l. A system for reproducing first, second, third and fourth audiosignals which are recorded in a single groove cut on a record disc,wherein a signal representative of the combination of a signalrepresentative of the sum of said first and second audio signals and asingle sideband signal with carrier of predetermined frequency modulatedby a signal representative of the difference of said first and secondaudio signals is recorded on one wall of the record groove and a signalrepresentative of the combination of a signal representative of the sumof said third and fourth audio signals and a single sideband withcarrier of said predetermined frequency modulated by a signalrepresentative of the difference of said third and fourth audio signalsis recorded on the other wall of the record groove, and wherein thelower frequency limit of each of said single sideband with carriersignals is higher than the upper frequency limit of said sum signals,said reproducing system comprising:

pickup means for generating a first electrical signal in response to thesignals recorded on the one wall of the groove and for generating asecond electrical signal in response to the signals recorded on theother wall of the groove, first low pass filter means to which saidfirst electrical signal is applied for deriving the signalrepresentative of the sum of said first and second audio signals andfirst high pass filter means to which said first electrical signal isapplied for deriving the single sideband with carrier which is modulatedwith the signal representative of the difference of said first andsecond audio signals, second low pass filter means to which said secondelectrical signal is applied for deri ing the signal representative ofthe sum of said third and fourth audio signals and second high passfilter means to which said second electrical signal is applied forderiving the single sideband with carrier which is modulated with thesignal representative of the difference of said third and fourth audiosignals,

first amplitude detector means for demodulating the single sideband withcarrier derived by the first high pass filter means to thereby reproducethe sig' nal representative of the difference of said first and secondaudio signals,

second amplitude detector means for demodulating the modulated carrierwave derived by the second high pass filter means to thereby reproducethe signal representative of the difference of said third and fourthaudio signals,

first matrix means responsive to the sum signal derived by said firstlow pass filter means and the reproduced signal from said firstamplitude detector means for separately reproducing the original firstand second audio signals, and

second matrix means responsive to the sum signal de rived by said secondlow pass filter means and the reproduced signal from said secondamplitude detector means for separately reproducing the original thirdand fourth audio signals.

2. The reproducing system as defined in claim 1 further comprising fourloudspeakers for generating sound in response to respective ones of thereproduced first, second. third and fourth audio signals, and

wherein said loudspeakers are arranged with respect to the listener sothat the first and second loudaudio signals are placed at the front andon the other side and at the rear and on the other side, respectively.

1. A system for reproducing first, second, third and fourth audiosignals which are recorded in a single groove cut on a record disc,wherein a signal representative of the combination of a signalrepresentative of the sum of said first and second audio signals and asingle sideband signal with carrier of predetermined frequency modulatedby a signal representative of the difference of said first and secondaudio signals is recorded on one wall of the record groove and a signalrepresentative of the combination of a signal representative of the sumof said third and fourth audio signals and a single sideband withcarrier of said predetermined frequency modulated by a signalrepresentative of the difference of said third and fourth audio signalsis recorded on the other wall of the record groove, and wherein thelower frequency limit of each of said single sideband with carriersignals is higher than the upper frequency limit of said sum signals,said reproducing system comprising: pickup means for generating a firstelectrical signal in response to the signals recorded on the one wall ofthe groove and for generating a second electrical signal in response tothe signals recorded on the other wall of the groove, first low passfilter means to which said first electrical signal is applied forderiving the signal representative of the sum of said first and secondaudio signals and first high pass filter means to which said firstelectrical signal is applied for deriving the single sideband withcarrier which is modulated with the signal representative of thedifference of said first and second audio signals, second low passfilter means to which said second electrical signal is applied forderiving the signal representative of the sum of said third and fourthaudio signals and second high pass filter means to which said secondelectrical signal is applied for deriving the single sideband withcarrier which is modulated with the signal representative of thedifference of said third and fourth audio signals, first amplitudedetector means for demodulating the single sideband with carrier derivedby the first high pass filter means to thereby reproduce the signalrepresentative of the difference of said first and second audio signals,second amplitude detector means for demodulating the modulated carrierwave derived by the second high pass filter means to thereby reproducethe signal representative of the difference of said third and fourthaudio signals, first matrix means responsive to the sum signal derivedby said first low pass filter means and the reproduced signal from saidfirst amplitude detector means for separately reproducing the originalfirst and second audio signals, and second matrix means responsive tothe sum signal derived by said second low pass filter means and thereproduced signal from said second amplitude detector means forseparately reproducing the original third and fourth audio signals. 2.The reproducing system as defined in claim 1 further comprising fourloudspeakers for generating sound in response to respective ones of thereproduced first, second, third and fourth audio signals, and whereinsaid loudspeakers are arranged with respect to the listener so that thefirst and second loudspeakers which respectively reproduce the first andsecond audio signals are placed at the front and on one side and at therear and on the one side, respectIvely, and the third and fourthloudspeakers which respectively reproduce the third and fourth audiosignals are placed at the front and on the other side and at the rearand on the other side, respectively.